1. Field of the Invention
The present invention relates to a process for recovering from dyestuff-containing information recording media having a dyestuff-containing layer the dyestuff contained in said dyestuff-containing layer, and also to a solvent used for dyestuff recovery.
2. Description of the Related Art
Being capable of high-density recording, optical information recording media are widely used to record music, images, and information. Among other things, compact disks (CD) and discoid recording media of similar type are in general use. Under this situation, the recycling of discoid recording media has become an issue under discussion.
Optical disks now on the market are mostly of the read-only type such as music CD. They are of laminate structure, consisting of a substrate of polycarbonate or the like and a reflecting film of aluminum or the like and a protective film of acrylic resin or the like which are formed consecutively on the substrate (although not shown).
Optical disks of this structure have induced one to devise several methods for their recycling. Among them is crushing disks whole, followed by molding. A disadvantage of this method is that the components of the reflecting film and protective film enter the crushed product, making it impossible to recover the reflecting film only. In addition, this method gives rise to opaque molded products which merely find reuse in limited applications.
By contrast, another method, which is intended to reuse the substrate resin, is by immersing disks in an acidic or alkaline solution so as to dissolve the reflecting film (aluminum) and separate the protective film from the substrate. This method offers an advantage of giving rise to a recovered substrate resin which is transparent and finds reuse in a comparatively wide range of applications, although there is room for improvement in the recovery of the reflecting film.
In the meantime, the family of CD on the market includes recordable CD as well as the above-mentioned read-only CD.
An example of such recordable CD is one which is composed of a substrate of resin such as polycarbonate and laminate layers consecutively formed thereon, including a recording layer with a dyestuff such as cyanine dye, and a reflecting film of gold or aluminum, and a protective film of acrylic resin or the like.
To be more specific, commercially available are dye-based optical disks of write-once type, which are referred to as CD-R (Compact Disk Recordable). Akin to CD-R is DVD-R (Digital Video Disk Recordable) for high-density recording, which is formed by sticking two pieces of CD-R together. These disks are usually capable of writing only once.
In other words, these dye-based optical disks of write-once type have a recording layer which contains a dyestuff. They permit the recording and reproduction of information owing to their property that the recording layer changes in state according as the dye undergoes irreversible change.
In the dye-based optical disk of write-once type as mentioned above, the dyestuff partly changes in its state upon recording (or writing). The rate of change, however, is usually about 20%, with the rest (about 80%) remaining unchanged. In fact, about 80% of the total amount remains unchanged even in the case where the recording pattern is used up.
Since the dyestuff used for the optical disk of this type is expensive and valuable, it is desirable for cost reduction and effective resource use to recover and reuse it efficiently.
One known method for recycling dye-based optical disks resorts to buffing to separate the substrate resin from the reflecting film. A disadvantage of this method is that buffing mechanically removes the dyestuff together with the reflecting film, making its recovery difficult.
At present, the dye-based optical disk of write-once type employs an organic dyestuff (such as cyanine dye and phthalocyanine dye) in its dye-containing layer. In one instance, the dyestuff is mixed with a singlet oxygen quencher.
A quencher is a kind of stabilizer to protect the dyestuff from decomposition. Dyestuff is usually subject to oxidation and decomposition, which decrease its effect, in the presence of oxygen and light. A common way to protect dyestuff from deactivation (oxidation and decomposition) by oxygen is to incorporate a quencher into the dyestuff-containing layer beforehand.
Because the deactivation of dyestuff takes place in the presence of oxygen and light, no singlet oxygen quencher is necessary if the dyestuff-containing layer is closely sealed and completely isolated from oxygen by the reflecting film and protective film immediately after the spin coating and drying of the dyestuff-containing material. In fact, there exists a dye-based optical disk containing no quencher.
Dyestuff is subject to photo-oxidative decomposition, as mentioned above. However, it is practically very difficult to completely shut out oxygen in the course of dyestuff recovery which involves steps of dissolving the dyestuff-containing layer, concentrating the resulting solution, and separating the desired dyestuff by column chromatography. Moreover, there is an instance where the dyestuff is incorporated with a photosensitizer which would promote deactivation upon heating for concentration. Especially, an efficient recovery of dyestuff from the dyestuff-containing layer free from quencher is difficult.
Even though the dyestuff-containing layer is incorporated with a singlet oxygen quencher beforehand, it is difficult to prevent the entrance of oxygen, thereby protecting the dyestuff from oxidation and decomposition completely, in various steps in the course of dyestuff recovery. In other words, the recovery of dyestuff from the dyestuff-containing layer with a singlet oxygen quencher is also limited in efficiency.